This invention relates to a catalyzed hydrocarbon conversion process, and particularly to a process involving catalysis of the reaction of hydrogen with organosulfur or organonitrogen compounds to yield a denitrogenated and/or desulfurized product. More particularly, the invention relates to a process for removing color body compounds from hydrocarbon streams and is particularly concerned with the process for removing color body compounds from diesel fuel feedstocks to provide a relatively colorless diesel fuel.
In the refining of liquid hydrocarbons derived from mineral oils and other sources, it is often desirable to subject the liquid hydrocarbon or fraction thereof to hydrotreating. Hydrotreating is a refining process wherein liquid hydrocarbons are reacted with hydrogen. Hydrotreating is often employed to reduce the concentration of organosulfur and organonitrogen compounds in hydrocarbons. Reducing the concentration of organonitrogen and organosulfur produces a product hydrocarbon which, when eventually combusted, results in reduced air pollutants of the forms NO.sub.x and SO.sub.x.
In general, the hydrotreating of a organonitrogen and/or organosulfur-containing feedstock is carried out by contacting the feedstock with hydrogen at elevated temperatures and pressures and in the presence of a suitable catalyst so as to convert the organonitrogen to ammonia and the organosulfur to hydrogen sulfide.
Recent legislation has increased the demands for refiners to reduce the content of environmentally undesirable sulfur and nitrogen compounds in solvents and fuel products such as diesel fuels. Catalytic hydrotreating is a useful refining process for such reduction. However, this objective becomes more difficult to meet as heavier crude oils are processed and more cracked stocks, coker gas oils, and light cycle oils obtained from fluid catalytic cracking processes are utilized to obtain such fuel and solvent products--since such feedstocks tend to have increased sulfur or nitrogen content. The increased sulfur or nitrogen contents in such feedstocks typically require the refiner to more severely hydrotreat the feedstocks, i.e., using increased temperature, pressure, hydrogen throughput, lower space velocity, and the like.
It is generally recognized that it is difficult to hydrotreat a feedstock containing diesel fuel and maintain a very good product color. As reactor temperatures of a hydrotreater are increased to meet product specifications for sulfur, nitrogen, etc., the color of the hydrotreated diesel product darkens, (i.e., degrades). In other words, the higher the temperature at which a diesel fuel feedstock is hydrotreated, the darker the color of the liquid hydrocarbon effluent product.
Thus, it is desirable to develop a process whereby color is effectively reduced in a feedstock while minimizing the cost or expenditure of energy to achieve such results. Two relatively expensive approaches to minimizing energy requirements include (1) an increase in the amount or volume of the catalyst, or (2) an increase in the pressure of the reactor-both approaches involving considerable capital expenditure.
Accordingly, the present invention provides a process for removing color bodies from hydrocarbon feedstreams during catalytic hydrotreating, and particularly during severe conditions of hydrotreating. The invention further provides a catalytic process for removing color bodies from feedstreams while utilizing relatively small catalyst and reactor volumes and still maintaining relatively low reactor pressures.